Photomasking using UV radiation as a light source achieves high resolution and productivity in the manufacture of ICs by lithography. This technique is especially adapted to the production of wafers or chips of higher packing density. "Deep" UV radiation is commonly employed as a light source because the resolution is increased as the wavelength of light is decreased. Because high resolution is achieved by photomasking, the smallest contaminant on the image surface of the photomask reduces the accuracy of the etched image to yield a defective product. The contaminant could be removed by cleaning, but this operation itself is deleterious to the photomask and shortens its service life.
Conventionally, the image on the photomask is protected from contaminants in the air by a thin resin film placed in the optical path on the image surface of the photomask. Any contaminants in the air build up on the surface of the thin resin film instead of on the image surface of the photomask. If the thickness of the resin film and the distance between the resin film and the image on the photomask remain the same at every point of the film, the harmful effect of any contaminants on the thin film on the surface of the photoresist is avoided so as to provide a faithful pattern on the resist after exposure. The thin film for protecting photomasks is therefore very useful in the semiconductor industry, but no commercial method for producing such protective film has yet been established.